Austenitic heat-resistant nickel-base alloy

ABSTRACT

Austentic heat-resistant nickel-base alloy comprising (in % by mass) 0.03-0.1% of C, 28-32% of Cr, 0.01-&lt;0.5% of Mn, 0.01-&lt;0.3% of Si, 0.01-&lt;1.0% of Mo, 2.5-3.2% of Ti, 0.01-&lt;0.5% of Nb, 0.01-&lt;0.5% of Cu, 0.05-&lt;2.0% of Fe, 0.7-1.0% of Al, 0.001-&lt;0.03% of Mg, 0.01-&lt;1.0% of Co, 0.01-0.10% of Hf, 0.01-0.10% of Zr, 0.002-0.02% of B, 0.001-0.01% of N, max. 0.01% of 5, max. 0.005% of Pb, max. 0.0005% of Bi, max. 0.01% of Ag, balance Ni and minor components due to the production method, where the sum of Ti +Al is from 3.3 to 4.3%, the sum of C+(10×B) is from 0.05 to 0.2%, the sum of Hf+Zr is from 0.05 to 0.15%, the Ti/Al ratio is &gt;3 and Zr/Hf is 0.1 to 0.5.

The invention relates to an austenitic heat-resistant nickel-based alloy.

BACKGROUND OF THE INVENTION

In the “Proceedings” for Diesel Engine Combustion Chamber Materials for Heavy Fuel Operation, 1990, the Institute of Marine Engineers provides a summary of the current prior art and the intensive research and development that has been performed in the preceding years in the field of valve materials. According to this, primarily Alloy 80 A, having (in mass %) 0.08% C, 19.5% Cr, 75% Ni, 1.4% Al, and 2.4% Ti has established itself for this application.

In some cases Alloy 81, having (in mass %) 0.5% C, 30% Cr, 66% Ni, 0.9% Al, and 1.8% Ti was also used. Occasionally these alloys are used for basic materials for valves, the valve seat section also being coated with a wear-resistant material as is described for instance in EP-B 0521821. This publication provides the chemical composition for the basic material as follows (in mass %): 0.04-0.10% C, ≦1.0% Si, s 0.2% Cu, ≦1.0% Fe, ≦1.9% Mn, 18-21% Cr, 1.8-2.7% Ti, 1.0-1.8% Al, ≦2.0% Co, ≦0.3% Mo, B, Zr, and the remainder nickel. Furthermore, a variant of this alloy is also presented inter alia with 29-31% Cr.

At the current usage temperatures of less than 750° C., Alloy 80 A was distinguished by a longer service life in LCF trials and better wear resistance, while, due to its better corrosion resistance, Alloy 81 was tested under conditions like those that would be encountered for instance in diesel engines for ships. Each of these alloys has its particular advantages, but neither satisfies all of the requirements for mechanical and corrosive properties. A remedy using an additional coating involves additional undesired production and material costs. Powder metallurgical production is also unfavorable from a cost standpoint. Such costs should be avoided to the greatest extent possible.

Both U.S. Pat. No. 6,139, 660 and U.S. Pat. No. 6,039,919 relate to this; they describe an alloy having the following composition (in mass %) for inlet and exhaust valves in diesel engines: ≦0.1% C, ≦1.0% Si, 0.1% Mn, ≧25-≦32.2% Cr, ≦3% Ti, ≦1-2% Al, remainder Ni. But this alloy does not provide adequate hot corrosion resistance, either. In addition, there is the fact that future more powerful engines, such as diesel ship engines, will be operated at temperatures up to about 850° C., which also places high demands on the valve material, especially when the service life is to be maintained and no additional maintenance work is desired.

Known from DE-C 101 23 566 is an austenitic, heat-resistant nickel-based alloy that has the following composition (in mass %): 0.03-0.1% C, max. 0.005% S, max. 0.05% N, 25-35% Cr, max. 0.2% Mn, max. 0.1% Si, max. 0.2% Mo, 2-3% Ti, 0.02-1.1% Nb, max. 0.1% Cu, max. 1% Fe, max. 0.08% P, 0.9-1.3% Al, max. 0.01% Mg, 0.02-0.1% Zr., max 0.2% Co, the sum of Al+Ti+Nb being ≧3.5%, the remainder being Ni and process-related impurities. The alloy is characterized by additions of (in mass %) 0.001-0.005% B, 0.01-0.04% Hf, and 0.01-0.04% Y.

The underlying object of the invention is to provide a material that is hot corrosion resistant up to temperatures of 850° C. and that has mechanical properties that are not inferior to those of Alloy 80 A.

SUMMARY OF THE INVENTION

This object is attained using an austenitic heat-resistant nickel-based alloy having (in mass %):

-   0.03-0.1% C -   28-32% Cr -   0.01-≦0.5% Mn -   0.01-≦0.3% Si -   0.01-≦1.0% Mo -   2.5-3.2% Ti -   0.01-≦0.5% Nb -   0.01-≦0.5% Cu -   0.05-≦2.0% Fe -   0.7-1.0% Al -   0.001-≦0.03% Mg -   0.01-≦1.0% Co -   0.01-0.10% Hf -   0.01-0.10% Zr -   0.002-0.02% B -   0.001-0.01% N -   max. 0.01% S -   max. 0.005% Ph -   max. 0.0005% Bi -   max. 0.01% Ag -   remainder Ni and process-related impurities, -   the sum of Ti+Al being between 3.3 and 4.3%, -   the sum of C+(10×B) being between 0.05 and 0.2%, -   the sum of Hf+Zr being between 0.05 and 0.15%, -   Ti/Al being>3, and -   Zr/Hf being=0.1-0.5%

Such hot corrosion resistant materials attain mechanical properties that are not inferior to those of Alloy 80 A. In this regard the inventive material is generally suitable for use as a valve material and especially can be employed for future generations of diesel ship engines in the temperature range up to a maximum of 850° C.

DETAILED DESCRIPTION OF THE INVENTION

Table 1 provides as examples the chemical composition of two inventive examples E1 and E2. Two typical analyses of the commercial alloys Alloy 80 A and Alloy 81 are provided in order to enhance the comparison.

Analyses of alloys E1 and E2 came from a series of laboratory melts, which were melted in 10 kg blocks in a vacuum induction oven, then were hot rolled and were solution annealed for two hours at 1180° C. in air with subsequent water quenching. The alloys are hardened in two additional annealings:

-   6 hours at 850° C. with air cooling followed by -   4 hours at 700° C. with air cooling.

The alloys differ in their content of the elements under discussion so that evaluation of their mechanical properties and their behavior in the corrosive medium led to the inventive analysis.

TABLE 1 Chemical composition of the inventive alloys E1 and E2 compared to Alloy 80A and Alloy 81 Element Alloy 80A Alloy 81 E1 E2 Ni Remainder Remainder Remainder Remainder Cr 19.5 28.4 29.1 31 Fe 0.13 0.09 0.1 1.7 Ti 2.25 2.1 2.8 3.1 Al 1.45 1.13 0.85 0.75 C 0.041 0.07 0.03 Mn 0.09 0.01 0.01 0.2 Si 0.20 0.04 0.02 0.1 Nb 0.001 <0.01 0.04 0.01 Mo 0.008 0.01 0.01 0.02 Cu 0.004 0.01 0.01 0.01 Mg 0.002 <0.001 0.001 0.005 S 0.004 0.003 0.002 P 0.002 0.002 0.002 N 0.002 0.006 0.0015 Hf 0.04 0.06 Co 0.039 0.01 0.01 0.3 B 0.003 0.003 Zr 0.02 0.02 0.04 Ti + Al 3.7 3.23 3.75 3.85 C + (10 × B) 0.1 0.06 Hf + Zr 0.06 0.10 Ti/Al 1.55 1.86 3.29 4.13 (Mass %)

Since one object of the invention was to attain heat resistance comparable to Alloy 80 A at the usage temperature, tensile strength and yield point were measured at 600° C. and 800° C. Table 2 demonstrates that Alloy 80 A is comparable and even stronger at 600° C. The alloys are comparable at 800° C.

TABLE 2 Tensile strength and yield point of E1 and E2 compared to Alloy 80 A at 600° C. and 800° C. 600° C. 800° C. Alloy R_(m)/MPa R_(p0.2)/MPa R_(m)/MPa R_(p0.2)/MPa E1 1053 738 636 552 E2 1062 690 617 573 Alloy 80 A 851 646 594 546

To investigate corrosion behavior, first experiments were performed in the laboratory in synthetic oil ash having the following composition:

40% V₂O₃+10% NaVO₃+20% Na₂SO₄+15% CaSO₄+15% NiSO₄.

The atmosphere was air with an SO₂ content of 0.5%. The specimens were aged both at 750° C. and 850° C., in each case for 20 hours, 100 hours, and 400 hours. For 400 hours of aging, the ash was renewed after 100 hours, 200 hours, and 300 hours in order to maintain corrosiveness. The depth of the internal corrosion could be reliably measured in the laboratory trials.

The corrosion investigations of the ship's diesel valve itself can be considered more reliable because they can be evaluated better and because they also take erosive effects into account. Specimens from each laboratory melt and, for comparison purposes, also from Alloys 81 and 80 were used in a diesel valve for ships. This diesel valve for ships ran more than 3000 hours in the main engine of an ocean-going vessel. Then the specimens were taken from the valve and the corrosion was investigated using metallography. This made it possible to distinguish material loss, layer thickness, and internal corrosion from one another in detail.

The investigations found the following relationships between corrosion behavior and the content of the individual alloy elements.

Cr: Cr content must be as high as possible from a corrosion standpoint. However, 32% is a reasonable metallurgical upper limit. The difference between the alloy variants with approx. 30% Cr and those with 20% Cr clearly demonstrated this. In the best case, corrosion in the former alloys is only half that of the latter alloys. In extreme close-ups, the specimens tested in the valve that have a Cr content of 30% have a cobblestone appearance that shows up in the polished micrograph sections as a wavy specimen surface and is a sign of only moderate corrosion wear. In contrast, the Cr-poor specimens already demonstrate significant symmetrical spalling. 28-31% Cr is more preferred and 29-31% Cr is most preferred.

Ti, Al: A Ti:Al ratio of >3 results in better corrosion resistance than a lower Ti:Al ratio. This is attributed to the formation of a Ti-rich margin between the outer oxide layer and the area of interior sulfidation with high Ti contents. Aluminum and titanium have a positive effect on heat resistance by forming a γ′ phase. The sum of elements Al+Ti should preferably be between 3.5 and 4.3%, more preferably 3.5-4.2%. It is more difficult to hot-form the material if the total content of these elements is too high. As to Ti content, 2.8-3.2% is more preferred and 2.8-3.0% is most preferred.

Si: The investigations demonstrated that silicon does not have a positive effect on corrosion properties and should be a maximum of 0.5%, less than 0.1% is better.

Nb: The niobium-alloyed specimens in principle have the thinnest corrosion layer, but this does not have any effect on material loss itself. Since a thick corrosion layer has a protective effect against the progress of corrosion, the Nb content should be limited to a maximum of 0.5%. Moreover, Nb influences material strength due to its high solubility in the y′ phase. The Ti and Al contents do not have to be adapted at Nb contents less than 0.5%.

B, C: The addition of boron in contents preferably of 0.002-0.01% improves corrosion resistance such that the interior sulfidation, which preferably runs along the grain boundaries, is reduced and therefore all of the corrosion is reduced. Most preferred are boron contents of 0.002-0.005%. Carbon preferably forms Cr carbides on the grain boundaries. Boron forms borides, which contribute to stabilizing the grain boundaries and thus to long-term strength. In particular the Cr carbides that form lead to Cr depletion in the vicinity of the grain boundaries, which is why corrosion progresses at an accelerated pace when the C content is to high. In addition, carbides and borides must not cover the grain boundaries too heavily because then they sharply reduce the ductility of the material as hard deposits. It was found that as a compromise the sum of C+(10×B) should preferably not exceed 0.1%. The aforesaid sum is most advantageously about 0.08%. A preferred range is 0.05-0.1% and a more preferred range 0.05-0.08%.

Hf: Hafnium is frequently added to improve resistance to high temperature oxidation and evidently also has a positive effect on the resistance of the specimens in vanadium ash and SO₂ atmospheres. Moreover, Hf also changes the grain boundary properties, forming carbide or carbosulfide. A Hf content that is too high should be avoided because otherwise hot forming is no longer possible. A preferred concentration range is therefore 0.01-0.08% and most preferred between 0.02 and 0.08%, particularly 0.05%. The effect of Hf on grain boundaries is comparable to the effect of Zr, and this is why Hf+Zr may be <0.10%. More particularly, Hf+Zr is preferably 0.05-0.15% and most preferably 0.05-0.10%.

Zr: Zirconium forms carbosulfides, which have a positive effect on long-term strength and also contribute to hot corrosion resistance by bonding with sulfur. It has been demonstrated that a Zr content between 0.01 and 0.05% is preferable. A Zr content in the area of 0.02% is especially preferred.

Co: Co is an element that principally increases resistance to sulfur-containing media. However, it is also very expensive, so Co is not added to the alloy. Nevertheless, because of the constituents in the materials used, the Co content may reach up to 2% without this resulting in higher costs.

Fe: The element iron occurs inter alia as an accompanying element. Reducing the iron content clearly to less than 1% increases costs because higher quality raw materials would have to be selected. Given an Fe content limited to 3%, preferably 2%, it is not necessary to anticipate that corrosion resistance will clearly be worse, nor that costs will be too high. Still, an Fe content of less than 1% should be sought.

Mn: The conditions mentioned for Fe also apply for Mn, it being possible to reduce Mn content to less than 1% without great difficulty.

Although the effects of the various elements on corrosion behavior and heat resistance frequently oppose one another, in the alloys El and E2 it was possible to find compositions that simultaneously satisfy the stated requirements for high temperature corrosion behavior and heat resistance at temperatures in the range between 600° C. and 850° C. The good corrosion resistance can be attributed to the addition of the reactive elements, such as hafnium and zirconium, without exceeding the selected optimum (0.05-0.10%). Higher contents strengthen the corrosion directed into the material. Limiting the carbon content <0.1% and manganese <1% also contribute to corrosion resistance. For heat resistance it has proved particularly favorable when aluminum and titanium are added, their total content preferably being in the range between 3.5 and 4.3%, more preferably 3.5-4.2%, as stated. This heat resistance renders coating the valve seat section unnecessary, so that there is a savings in production costs.

The alloy can be produced using the conventional methods of a melting operation, melting in a vacuum with subsequent remelting by electroslag process is advantageously reasonable. There is formability for producing rods for further processing to create valves, such as for instance diesel valves for ships.

The inventive alloy is also suitable in particular for producing valves for large diesel engines in general, that is, for instance also for large diesel engines that are employed in stationary systems for obtaining current. 

1. Austenitic heat-resistant nickel-based alloy having (in mass %): 0.03-0.1% C 28-32% Cr 0.01-≦0.5% Mn 0.01-≦0.3% Si 0.01-≦1.0% Mo 2.5-3.2% Ti 0.01-≦0.5% Nb 0.01-≦0.5% Cu 0.05-≦2.0% Fe 0.7-1.0% Al 0.001-≦0.03% Mg 0.01-≦1.0% Co 0.01-0.10% Hf 0.01-0.10% Zr 0.002-0.02% B 0.01-0.01% N max. 0.01% S max. 0.005% Pb max. 0.0005% Bi max. 0.01% Ag remainder Ni and process-related impurities, the sum of Ti+Al being between 3.3 and 4.3%, the sum of C+(10×B) being between 0.05 and 0.2%, the sum of Hf+Zr being between 0.05 and 0.15%, and Ti/Al being >3.
 2. Alloy in accordance with claim 1 that includes (in mass %) 28-31% Cr.
 3. Alloy in accordance with claim 1 or 2 that includes (in mass %) 29-31% Cr.
 4. Alloy in accordance with claims 1 through 3, that includes (in mass %) 2.8 3.2% Ti.
 5. Alloy in accordance with claims 1 through 4 that includes (in mass %) 2.8-3.0% Ti.
 6. Alloy in accordance with claims 1 through 5 that includes boron as an additive (in mass %) 0.002-0.01%, in particular 0.002-0.005%.
 7. Alloy in accordance with claims 1 through 6 in which the sum of C+(10×B) is 0.05 to 0.1%, and in particular is between 0.05-0.08%.
 8. Alloy in accordance with claims 1 through 7 in which the Zr content is set between 0.01 to 0.05%.
 9. Alloy in accordance with claims 1 through 8 in which the Hf content is set between 0.01 to 0.08%.
 10. Alloy in accordance with any of claims 1 through 9 in which the following relationship is true: Zr/Hf=0.1-0.5%
 11. Alloy in accordance with any of claims 1 through 10, characterized in that Ti/Al is between 3.3 and 4.2.
 12. Use of the alloy in accordance with any of claims 1 through 11 as a valve material, in particular for valves that can be used in diesel engines.
 13. Use of the alloy in accordance with any of claims 1 through 11 as a valve material for valves that can be used in diesel engines on ships in the temperature range up to 850° C.
 14. Valve, in particular for a large diesel engine, that at least partially comprises an alloy in accordance with any of claims 1 through
 11. 